If it helps, feel free to adopt https://certmon.toolforge.org/ which was quickly thrown together in an attempt to help Wikimedia to improve its monitoring. See source code and the metrics endpoint for Prometheus monitoring. Feel free to fork, send pull requests, whatever. Please do tell if you end up using it, I’m quite curious. If it’s useful, my personal preference would be that you’d clone the repo into a better place (perhaps a Phabricator project) and run it yourself, so the Wikimedia SRE team could change things without me getting involved.